A trephine is a surgical instrument having a cylindrical blade. Particular types of trephine are used in ophthalmic surgery to cut buttons from donor corneal grafts, and also to cut away diseased sections of a patient's cornea.
Typical corneal trephines are known, for example, from U.S. Pat. No. 4,319,575 and U.S. Pat. No. 2,473,968, the disclosures of which are hereby incorporated into the present application by reference.
A more recent development in trephine technology comprises an outer, generally cylindrical casing with an annular base and an inner, cylindrical tube of slightly smaller diameter than the outer casing and also having an annular base, slightly recessed from the annular base of the outer casing. This allows the base of the casing to be placed on the curved cornea, with the recessed base of the tube also resting on the epithelium as a result of the convex curvature of the cornea. When a vacuum is applied to the cylindrical space between the outer casing and the inner tube, the casing becomes attached to the epithelium by suction, thereby preventing movement between the casing and the cornea. A cylindrical trephine blade is mounted inside the inner tube and provided with a screw mechanism so as to allow the blade to be raised and lowered within the inner tube. A spoked wheel is provided at an end of the trephine remote from the base so as to allow the amount the blade is raised and lowered to be determined by a number of turns or fractions of turns of the spoked wheel.
In use, the trephine is examined under an operating microscope and the spoked wheel is turned until the blade of the trephine is aligned with the base of the inner tube, this being the zero position. The blade is then retracted by turning the spoked wheel anticlockwise so as to ensure that the blade does not touch the cornea when the vacuum trephine assembly is placed on the epithelium with both the base of the casing and the base of the inner tube contacting the epithelial surface of the cornea. If the blade is not sufficiently retracted, the blade is forced into the cornea.
A vacuum is then applied to the cylindrical space between the casing and the inner tube, for example by using a syringe with a flexible tube connected to the annular space.
Once a good vacuum seal has been obtained and the assembly is fixed to the cornea by suction, the spoked wheel is rotated clockwise until the blade touches the cornea (this will generally be slightly behind the zero position due to the convex curvature of the cornea), and cutting then starts by continuing to rotate the spoked wheel a desired number of turns. In currently available embodiments of this type of vacuum trephine, each complete revolution of the spoked wheel raises or lowers the blade of the trephine by approximately 0.25 mm relative to the casing and the inner tube. At the desired depth of cut, the vacuum is released by operating the syringe appropriately, and the trephine is then lifted from the patient's eye.
While operation of the vacuum trephine has been described with reference to a living patient, it may also be used to cut a button from a donor corneal graft harvested from a cadaver and mounted on an artificial anterior chamber.
EP2191799 discloses a trephine apparatus with a transparent casing having an outer casing and an inner casing. A vacuum or partial vacuum may be applied to the space between the outer casing and the inner casing using a suction pump. This enables the trephine apparatus to be releasably attached to the surface of the eyeball. The outer and inner casings are formed from a non-opaque material to improve the user's visibility of the blade.
Some transparent casings may still obscure the user's visibility of the trephine blade.
Furthermore, there are difficulties with existing vacuum trephine devices relating to the fit of the vacuum trephine device on the eye. No two eyes are identical and therefore accommodating for the individual form of the eye can be challenging and costly. Again, this may be compensated for by increasing the strength of the vacuum in the vacuum chamber, but this can cause damage or scarring to the eye.